Various image display devices have been known as a device for projecting a two-dimensional image into a field of view of an observer, which propagates image light while repeatedly reflecting the light within a light guide plate, so as to emit the image light which is deflected in part toward the observer's side on one surface side of the light guide plate, to thereby expand an exit pupil. As a method of deflecting image light in the light guide plate, it has been known to use diffraction effects (see, for example, JP2013061480A (PTL 1)).